It is known to blow mold a container by blowing gas into a preform. The resulting container can then be filled. This, however, is a two-step operation.
It is also known to transform a preform into a container by filling it with liquid filling-material that is under sufficient hydraulic pressure. Essentially, the liquid filling material plays the role of the gas that would normally be blown into the preform.
When the foregoing procedure is used, the container must be completely filled in order to be entirely molded. As a result, it is necessary to remove some of the liquid filling-material to create a headspace before closing the container.
In certain cases, a stretching bar supports the preform as it expands into a container. If this stretch bar is removed from the container after filling, the volume displaced by the stretching bar becomes the volume of the headspace.